Heterodyne measurement of Coherent Transition Radiation (CTR) from Seeded Self-Modulation (SSM) in AWAKE

Transcription

1 Heterodyne measurement of Coherent Transition Radiation (CTR) from Seeded Self-Modulation (SSM) in AWAKE Falk Braunmueller, P. Muggli, M. Martyanov, F. Batsch, K. Rieger, A. Caldwell & AWAKE team 27 September rd European Advanced Accelerator Concepts Workshop Elba, Italy

2 Outline - Setup of heterodyne CTR-measurements - Measurement principle - Measurement processing - Main result: f CTR = f plasma (n Rb ) - Further results: Dependence of SSM on Rb-density gradient 2

3 SSM-Diagnostics via CTR preliminary Modulated p + -bunch Coherent transition f modulation (90-280GHz) OTR Courtesy: T.Haubold OAPmirror CTR F. Batsch, Poster session 19:30 preliminary Coupled into WR90 waveguide 15m transmission

4 SSM-Diagnostics via CTR Coherent transition f modulation (90-280GHz) Modulated p + -bunch Frequency: Heterodyne mixing OTR 15m transmission Laser-based Courtesy: T.Haubold Amplitude: Schottky diodes OAPmirror Waveguidebased

5 Diagnostic setup 3 Heterodyne receivers for CTR: End of transmission line - Laser-based mixing (last presentation) - WR8 / GHz: Radiometersystem new Beam splitter Mirror - WR3.4 / GHz: VDI-system from EPFL replaced by WR4.3/ GHz system Can detect 2 nd harmonics of f modulation June August 5

6 Measurement principle Signal: f RF ~260GHz Reference: f ref ~270GHz Intermediate frequency: Mixer f IF ~10-20GHz Expected signal (f IF =10GHz) f ref from frequency-multiplication of tunable local oscillator f ref = n harm f LO Also mixing with weaker parasitic reference frequencies f ref = n harm,1 f LO. (n harm,1= n harm +/-1, ) Confirm that signal on oscilloscope is from mixing with correct reference frequency: fix f IF = f RF n harm f LO n harm = f IF / f LO measured setting to be determined 6

7 CTR-signal from mixer Short signal, close to expected length Very precise Strong single-frequencycomponent (find via spectrogram) f IF 7

8 Data-selection Choice of useful data: Signal level large enough, e.g. > 40mV Use only prominent peaks : Significantly higher than other IFpeaks Previously: selection by eye (shot-to-shot variation of parameters) 8

9 CTR-analysis Fit f IF vs. f LO to check n harm In general, expected n harm =8 / 12 / 24 is confirmed (sometimes ambiguous) f RF = n harm f LO +/ f IF Average & standard deviation of f RF (here: 255.9GHz +/- 1.4GHz) Unclear if from change of CTR-freq. 9

10 Results of CTR-analysis Result: f CTR vs. n RB f CTR = f plasma (n Rb ) SSM with f CTR =f plasma as predicted Rb fully ionized Good match between fundamental & 2nd harmonics proof that correct n harm (f LO ) was chosen vapour Excellent fit result: parameters within 0.3% Error analysis incomplete Error bars: Std GHz Preliminary result (95% confidence of fit on mean) 10

11 CTR-amplitude (Standard dev.) (WR4-system) Amplitude increasing with beam-charge Preliminary result E CTR ~q, but: - SSM-amplitude affected in nontrivial way - Emission angle & coupling may be affected Promising for future analysis 11

12 f CTR -dependence on n Rb -gradient 10% gradient 138GHz 10 m Negative Gradient: f CTR =129GHz ~const. Preliminary result 131GHz % gradient 131GHz Moving average 134.5GHz 10 m Evolving interaction over several meters! Frequency increasing with positive gradient, but basically constant with negative gradient Explanation from SSM?

13 SSM-Dependence on n Rb -gradient No/Small gradient microbunches reach less far f RF ~f plasma (end) Preliminary result & possibly varying parameters ξ along bunch [a.u.] Gradient >5%: - Microbunches longer visible after seeding - f RF corresponds more to f plasma (end) longer interaction in plasma? Negative Gradient: f CTR =129GHz const. Check with simulations?! f plasma 10% gradient entry 131GHz GHz 10 m

14 Summary Several successful upgrades of heterodyne CTR setup Consistent results after data down-selection Very successful measurement of f CTR =f plasma (n Rb ), confirming full ionization + character of SSM Clear correlation between beam charge & signal amplitude Investigation of self-modulation physics: - f CTR =f plasma (n Rb,downstream) for positive n Rb -gradient - Longer persisting microbunches Analysis to be continued Preliminary results longer interaction? 14

15 Thanks for your attention! Acknowledgement ( GHz-system): Work supported by Requip, Sinergia and (No: /1), grants of the Swiss National Science Foundation, by the Ecole Polytechnique Federale de Lausanne (EPFL) and by Faculty of Basic Sciences of EPFL.

16 Additional slides

17 Analysis/Measurement To-Do-List - Apply criterion of prominent peak to all points - Analysis of signal amplitude: need to correlate with good shots from streak camera & two-screen halo-btv Frequency-variations correlated with alignment/ angle of p + -defocusing/? - Ratio of signal amplitudes V(2 nd harmonics)/v(fundamental) vs. p + charge idea: more non-linear stronger 2 nd harmonics? 17

18 Measurement principle f RF = n harm f LO +/ f IF to be determined known measured f RF =100GHz f RF =260GHz Find n harm by scanning f LO : n harm = f IF / f LO

19 Heterodyne Measurement Measure intermediate frequency (IF) between CTRsignal (RF) and known reference Reference signal from frequency-multiplied tunable local oscillator (LO) Waveguide Transmission of RF over 15m Small measurement bandwidth Good signal efficiency VDI heterodyne receiver from Swiss Plasma Center (SPC) at EPFL (Lausanne) ~10-20 GHz out RF in Reference

20 Waveguide Transmission Line - Detector behind shielding wall - 15m of overmoded waveguide WR90 (fundamental mode 8-12GHz) E-field polarization

21 Measurement principle f RF = n harm f LO +/ f IF to be determined known measured f RF =100GHz fixed f LO f RF =260GHz Single f RF with fixed f LO can give several f IF -signals n harm f LO = f RF Signal frequency must be constant to within 1-2GHz!